Method and apparatus for obtaining pressurized nitrogen by cryogenic separation of air

ABSTRACT

The distillation column system has a high-pressure column, a low-pressure column, a main condenser and a low-pressure-column top condenser. Feed air is cooled in a main heat exchanger and introduced into the high-pressure column. An oxygen-enriched liquid stream is withdrawn from the high-pressure column and introduced into the low-pressure column. A gaseous nitrogen stream is withdrawn from the high-pressure column, warmed in the main heat exchanger and withdrawn as gaseous pressurized nitrogen product. The high-pressure column has a barrier-plate section arranged immediately above the point at which the feed air is introduced. The oxygen-enriched liquid stream is withdrawn from the high-pressure column above the barrier-plate section. A purge stream is withdrawn below the barrier-plate section. The gaseous nitrogen stream, before being warmed in the main heat exchanger, is warmed in a counter-current subcooler in indirect heat exchange with the oxygen-enriched liquid stream from the high-pressure column.

The invention relates to a method for obtaining compressed nitrogen bycryogenic separation of air according to the preamble of Claim 1.

The method relates in particular to systems involving the withdrawal ofnitrogen product from the high-pressure column. The nitrogen product cancome from both columns, for example by gaseous nitrogen (GAN) beingpassed both directly out of the low-pressure column and out of thehigh-pressure column. Alternatively, at least a part of thelow-pressure-column nitrogen can be withdrawn in liquid form (LIN—liquidnitrogen), fed into the high-pressure column and drawn off therefrom asa GAN product. Such methods involving low-pressure-column LIN being“pumped back” into the high-pressure column are known from US 2004244417A1, FIG. 2, DE 19933557 or EP 1022530. In such processes, maincondensers and low-pressure-column top condensers are generally used,which are in the form of bath evaporators on their evaporation side.This represents the tried-and-tested evaporator form, in which inparticular no operational difficulties on account of volatile componentsthat are heavier than oxygen, for example propane, should be expected.However, in terms of energy, bath condensers are not optimal, becausethe hydrostatic level in the liquid bath leads to an increasedevaporation temperature.

The invention is based on the object of improving the method mentionedat the beginning and a corresponding apparatus in terms of energyconsumption and at the same time to allow safe operation of the system.

This object is achieved by all of the features of Claim 1.

The use of a forced-flow evaporator as low-pressure-column top condenserallows a particularly lower pressure difference between the evaporatingand the condensing stream with the same average temperature differenceas in a bath evaporator. This noticeably reduces the energy consumptionof the plant, for example by 3.2% at a product output pressure in thenitrogen of 10 bar, which corresponds to the high-pressure-columnpressure; if a further compression from 10 to 60 bar is also figured in,the energy saving is 2.2% of the total energy consumption.

However, the loss of the liquid bath above the low-pressure column isalso accompanied by the loss of the possibility of withdrawing a purgestream and discharging high-boiling components, in particular propane.In the invention, this is compensated in that a purge stream is drawnoff from the bottom of the high-pressure column. Above this withdrawal(and the infeed of feed air), a barrier-plate section is provided, whichretains the high-boiling components, in particular propane, in thebottom of the high-pressure column. The oxygen-enriched liquid streamfor the low-pressure column is withdrawn above the barrier-plate sectionand contains fewer high-boiling components and in particular virtuallyno propane any more. Even with two theoretical plates in thebarrier-plate section, given a propane content of 0.0075 ppm in the airdownstream of the air cleaner (with an exemplary assumption for propaneretention in the molecular sieve of the air cleaner of about 85%), 99.8%of the propane is removed with the purge stream. In the process, 84% ofthe N₂O is also separated out (relative to the N₂O quantity which passesthrough the air cleaner). The degrees of separation of other componentsare 69% for C₂H₆, 15% for C₂H₄ and about 2.5% for methane, which is lesscritical. “High-boiling components” are understood here to be substanceswhich have a higher evaporation temperature than oxygen.

In principle, the abovementioned measures can be used to ensure safeoperation of the plant. These measures are known per se from WO2016131545 A1, but are applied therein at a relatively high processpressure, which has the result that there is no pre-liquefaction, i.e.no liquefaction of the feed air upstream of the distillation; rather allthe air is introduced into the high-pressure column in gas form.

Overall, there are the following differences between the methodmentioned at the beginning according to US 2004244417 A1, FIG. 2 andthat of WO 201 61 31 545 A1:

US 2004244417 A1 WO 2016131545 A1 High air pressure, much greater thanTotal air is compressed only to high-pressure-column pressure.high-pressure-column pressure. 10% liquid production Gaseoushigh-pressure nitrogen as main product Large throttle stream (total airNo throttle stream without turbine air) over 232 Bath evaporatorForced-flow evaporator Residual-gas turbine makes only cold Residual-gasturbine makes only (does not drive a cold compressor) pressure (drives acold compressor)

The two methods have such different natures that there would be noquestion of combining them for an impartial person skilled in the art.

In US 2004244417 A1, on account of the relatively low pressure in theprocess (or relatively small pressure difference with the streamsemerging from the rectification system), the feed air also contains asmall liquid content during the feed into the high-pressure column -this would be the case even with very little liquid product beingobtained or purely gas operation. Therefore, a relatively large quantityof liquid would end up in the bottom of the high-pressure column, if theabovementioned measures (see also WO 2016131545 A1) were applied to oneof these methods. This quantity would be drawn off as a whole with thepurge stream and noticeably reduce the product yield or have a negativeeffect on the energy consumption of the plant.

For this reason, Claim 1 also contains a further feature, according towhich the gaseous nitrogen stream from the high-pressure column, beforebeing warmed in the main heat exchanger, is warmed in a counter-currentsubcooler in indirect heat exchange with the oxygen-enriched liquidstream from the high-pressure column. At first look, it appears unclearwhat this measure is supposed to have to do with the discharging of thehigh-boiling components. At any rate, it results in an increase in theenthalpy of the gaseous nitrogen stream at the inlet into the main heatexchanger. Since the difference in enthalpy of a balancing group remainsunchanged around the distillation column system (with unchanged productquantities and constant heat input from the environment), this causes atemperature increase at the cold end of the main heat exchanger. This isexperienced by the cooling feed air stream; therefore, it likewise hashigher enthalpy and a higher temperature than in the absence of warmingof the nitrogen in the counter-current subcooler. This increase inenthalpy prevents or reduces pre-liquefaction of the air and in manycases even has the result that the air stream is slightly superheated atthe inlet into the high-pressure column, i.e. its temperature isslightly above the dew point temperature; the temperature differencewith respect to the dew point in the case of superheating is for example1.4 K (in the method in which low-pressure-column LIN is “pumped back”into the high-pressure column and the nitrogen product is withdrawnprimarily from the high-pressure column). Thus, at the inlet into thehigh-pressure column, the feed air no longer contains any liquid and thepurge stream consists only of the reflux liquid, which exits thebarrier-plate section at the bottom.

With regard to a feed air quantity of 100 000 Nm³/h, this feed-airsuperheating, brought about by the warming of the pressurized nitrogenin the counter-current subcooler, is substantial and corresponds to aliquid production of about 1000 Nm³/h of liquid nitrogen. It is thuspossible for example for about 1% of the air quantity to be obtained asliquid product, without pre-liquefaction occurring; rather, the overallair quantity can be introduced into the high-pressure column in gasform. However, even at higher quantities of liquid nitrogen production(up to about 2% of the air quantity), there is still a certain amount ofsuperheating in the air stream, since with increasing liquid product,the feed air pressure is raised.

In a specific numerical example for a plant with 100 000 Nm³/h of feedair and a liquid production of less than 0.1% of the feed air quantity,in the following text, the invention is compared with an operating modein which the pressurized nitrogen is not passed through thecounter-current subcooler. If these measures are dispensed with, 96 600Nm³/h of air at 8.50 bar and a vapour content of 0.9966864 flow into thehigh-pressure column, that is to say 320 Nm³/h of air enter thehigh-pressure column in liquid form (pre-liquefaction). If, by contrast,the method is run in accordance with the invention, 96 105 Nm³/h are fedinto the high-pressure column at 8.55 bar with superheating of 1.405 K(with a similar size of the main heat exchanger or with the same averagetemperature in the main heat exchanger compared with the case withwarming of the pressurized nitrogen in the counter-current subcooler).Although this temperature difference with respect to the dew point seemsslight at first look, it has a very great effect on the process, becauseit relates of course to the entire air quantity flowing into thehigh-pressure column.

With the aid of the warming, according to the invention, of thepressurized nitrogen in the counter-current subcooler, the fraction ofair which is passed into the high-pressure column in liquid form istherefore reduced in a method in which more pre-liquefaction wouldotherwise occur. This “reduction” can go as far as zero or furthermoreresult in superheating of the air fed into the high-pressure column,i.e. in heating beyond the dew point. The invention does not relate tomethods in which pre-liquefaction already does not occur withoutintroduction of the pressurized nitrogen into the counter-currentsubcooler.

The described measure is relatively simple in terms of apparatus, butvery effective. It uses equipment that is required anyway, thecounter-current subcooler, and allows stable setting of the purge streamquantity which is withdrawn from the high-pressure-column bottom, withgood product yield and relatively low energy consumption. This resultsoverall in a particularly efficient method for obtaining pressurizednitrogen.

The operating pressures in the method according to the invention are:

Low-pressure column (at the top): for example 4.0 to 7.0 bar, preferably4.5 to 6.5 bar

High-pressure column (at the top): for example 7 to 12 bar, preferably 8to 11 bar

Low-pressure-column top condenser on the evaporation side: for example1.5 to 3.5 bar, preferably 1.9 to 3.2 bar

With the aid of the invention, pre-liquefaction can be reduced. Inindividual cases, decreased pre-liquefaction will still occur.Preferably, the pre-liquefaction is completely eliminated by theinvention, however; in other words, the feed air flows into thehigh-pressure column in a fully gaseous state under the dew point orwith slight superheating. “Slight superheating” is understood here tomean a temperature difference of at least 0.1 K, for example (dependingon liquid production) 0.1 K to 2.0 K, preferably 0.2 K to 1.8 K.

Preferably, the evaporation space operated as a forced-flow evaporatoris operated with an oxygen-rich liquid from the low-pressure column;this can come in particular from the bottom of the low-pressure column.The gas generated in the evaporation space of the low-pressure-columntop condenser is preferably warmed as residual gas to an intermediatetemperature in the main heat exchanger and subsequently expanded in awork-performing manner in a residual-gas turbine, and then reintroducedinto the main heat exchanger and warmed to around ambient temperature.As a result, cold for the method can be obtained economically.

The residual-gas turbine can be decelerated by an electric generator orby a compressor. The latter can compress for example the warmed expandedresidual gas or a part thereof.

The efficiency of the method can be increased further when theevaporation space of the main condenser is also in the form of aforced-flow evaporator.

The invention also relates to an apparatus according to Claim 10. Theapparatus according to the invention may be supplemented by apparatusfeatures which correspond to the features of individual, multiple or alldependent method claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention and further details of the invention are explained in moredetail in the following text by way of exemplary embodiments illustratedschematically in the drawings, in which:

FIG. 1a shows a first exemplary embodiment of the invention with agenerator turbine,

FIG. 1b shows a variant of FIG. 1a with a liquid nitrogen product beingobtained,

FIG. 2 shows a second exemplary embodiment of the invention with abooster turbine,

FIG. 3 shows a variant of FIG. 2, and

FIG. 4 shows a third exemplary embodiment of the invention withwithdrawal of GAN product from both columns.

In FIG. 1a , compressed and cleaned feed air arrives via line 1. Theinitial stages of an air compressor, a pre-cooler and an air cleaner,are not illustrated here and are embodied in a known manner in theexemplary embodiments. The air 1 is cooled almost to its dew point inthe main heat exchanger 2 and flows with a certain amount ofsuperheating into the bottom of the high-pressure column 4 of thedistillation column system via line 3. The distillation column systemalso has a main condenser 5, a low-pressure column 6 and alow-pressure-column top condenser 7. The two condensers are in the formof condenser-evaporators; their evaporation spaces are each operated asforced-flow evaporators.

According to the invention, the high-pressure column 4 has abarrier-plate section 8, which is arranged immediately above the pointat which the feed air 3 is introduced. It consists for example of one tofive, preferably of two to three conventional rectifier plates.Alternatively, a section with structured packing of for example one tofive, preferably two to three theoretical plates can also be used. Thissection retains high-boiling constituents of the air, in particularpropane, which are withdrawn with a purge stream 9A (Purge) from thebottom of the high-pressure column 4 and are removed therewith from thedistillation column system. To this end, the purge stream 9B can, asillustrated, be introduced in a warm waste stream 10.

Above the barrier-plate section 8, an oxygen-enriched liquid stream 11is withdrawn from the high-pressure column 4, cooled in acounter-current subcooler 12 and fed to the low-pressure column 6 at anintermediate point via line 13. This stream is virtually free of propaneand other high-boiling components. This then also goes for all otheroxygen-rich fractions in the low-pressure column, in particular for thebottoms liquid, which can be evaporated without risk both in the maincondenser 5 (via line 14) and in the low-pressure-column top condenser 7(via the lines 15 and 16). Complete evaporation can be carried outwithout problems in the low-pressure-column top condenser 7. With twotheoretical plates in the barrier-plate section, given a propane contentof 0.0075 ppm in the air downstream of the air cleaner (with anexemplary assumption for propane retention in the molecular sieve of theair cleaner of about 85%), 99.8% of the propane is removed with thepurge stream. In the process, 84% of the N₂O is also separated out(relative to the N₂O quantity which passes through the air cleaner). Thedegrees of separation of other components are 69% for C₂H₆, 15% for C₂H₄and about 2.5% for methane, which is less critical.

In the main condenser 5, a part 18 of the nitrogen tops gas 17 from thehigh-pressure column 4 is condensed. The liquid nitrogen 19 obtained inthe process is returned to the high-pressure column 4 as a recirculationflow. The low-pressure-column top condenser liquefies tops gas 20 fromthe low-pressure column 6. Liquid nitrogen 21 generated in the processis returned to the low-pressure column 6. A part thereof is immediatelydrawn off from the low-pressure column 6 again as a liquid nitrogenstream 22. (Alternatively, this stream could also be withdrawn directlyfrom the liquefaction space of the low-pressure-column top condenser 7).A pump 23 brings the liquid nitrogen stream 22 to approximatelyhigh-pressure-column pressure. The pressure liquid 24 is supplied to thetop of the high-pressure column 4 via the counter-current subcooler 12and line 25A/25B.

A gaseous nitrogen stream from the top of the high-pressure column 4 iswithdrawn via line 17/26A/26B and initially warmed according to theinvention in the counter-current subcooler 12. Subsequently, thenitrogen 27 is warmed in the main heat exchanger to around ambienttemperature and can be drawn off at 28 as gaseous pressurized nitrogenproduct under high-pressure-column pressure. In this example, however,it is compressed even further by one or for example two nitrogencompressors 29, 30 in each case with intermediate cooling orpostcooling, such that the final pressurized nitrogen product 31 (PGAN)exhibits a pressure of for example 120 or 150 bar here.

As a result of the evaporation of the low-pressure-column bottoms liquid16 in the low-pressure-column top condenser 7, a residual gas 32 isgenerated, which is initially warmed in the counter-current subcooler12. Subsequently, it flows via line 33 to the main heat exchanger 2, inwhich it is warmed to an intermediate temperature. Subsequently, it isexpanded in a work-performing manner in a residual-gas turbine 35 with abypass 37. The expanded residual gas is reintroduced in two parts intothe main heat exchanger and warmed to around ambient temperature. Afirst part 38 is fed as regeneration gas to the air cleaner via line 39.The rest 40 is discharged into the atmosphere (ATM) via line 10.

A part 41 of the tops gas of the low-pressure column 6 is discharged viathe lines 42 and 43 and through the counter-current subcooler 12 and themain heat exchanger 2 as sealing gas (Seal).

The line 44 shows the balancing group around the distillation columnsystem. It intersects the purge gas line 9A, the residual gas line 33and the sealing gas line 41 and especially the feed air line 3 and thepressurized nitrogen line 27 (illustrated in bold here). H_Luft meansthe enthalpy of the air stream, H_Prod the enthalpy of the productstreams, WPump the heat introduced by the pump 23.

FIG. 1b differs from FIG. 1a only in that a part 125C of the liquidnitrogen 22 warmed in the counter-current subcooler 12 is drawn off asliquid product LIN. Alternatively, the entire stream 25A can be guidedvia line 125C; the entire gaseous nitrogen product, which comes from thelow-pressure column 6, is then drawn off from the low-pressure column 6via line 41.

FIG. 2 differs from FIG. 1a only in that the turbine 35 is deceleratedby a compressor 236. The latter brings the part 39 of the warmedexpanded residual gas to the pressure that is required in order toemploy it as regeneration gas in the air cleaner. As a result, thepressure in the distillation column system and at the outlet of the aircompressor (not illustrated) can be reduced and the energy can be saveddirectly at the air compressor. For example, the pressure at the MAC islowered by about 500 mbar or even more in this case.

In FIG. 3, in contrast to FIG. 2, the entire expanded and warmedresidual gas 339 is compressed in the turbine-driven compressor 236. Afirst part 340 of the compressed residual gas is used, as in FIG. 2, asregeneration gas; the rest 341 is expanded in a throttle valve and letout into the atmosphere (Atm).

In the method in FIG. 4, in contrast to the preceding exemplaryembodiments, no liquid nitrogen is pumped out of the low-pressure column6 into the high-pressure column. Rather, the entire nitrogen product ofthe low-pressure column 6 is withdrawn directly in gas form via line41/42 and brought to high-pressure-column pressure in the warm state ina further nitrogen compressor 129. It can then be admixed to the productfrom the high-pressure column 28 or be drawn off separately via line 43.

Without further elaboration, it is believed that one skilled in the artcan, using the preceding description, utilize the present invention toits fullest extent. The preceding preferred specific embodiments are,therefore, to be construed as merely illustrative, and not limitative ofthe remainder of the disclosure in any way whatsoever.

In the foregoing and in the examples, all temperatures are set forthuncorrected in degrees Celsius and, all parts and percentages are byweight, unless otherwise indicated.

The entire disclosures of all applications, patents and publications,cited herein and of corresponding German application No. 102018000842.9,filed Feb. 2, 2018, are incorporated by reference herein.

The preceding examples can be repeated with similar success bysubstituting the generically or specifically described reactants and/oroperating conditions of this invention for those used in the precedingexamples.

From the foregoing description, one skilled in the art can easilyascertain the essential characteristics of this invention and, withoutdeparting from the spirit and scope thereof, can make various changesand modifications of the invention to adapt it to various usages andconditions.

1. Method for obtaining pressurized nitrogen by cryogenic separation ofair in a distillation column system which has a high-pressure column(4), a low-pressure column (6), and also a main condenser (5) and alow-pressure-column top condenser (7), which are both in the form ofcondenser-evaporators, wherein compressed and cleaned feed air (1) iscooled in a main heat exchanger (2) and is introduced (3) into thehigh-pressure column (4) at least mostly in gaseous form, anoxygen-enriched liquid stream (11, 13) is withdrawn from thehigh-pressure column (4) and introduced into the low-pressure column,and a gaseous nitrogen stream (17, 26A, 26B, 27) is withdrawn from thehigh-pressure column (4), warmed in the main heat exchanger (2) anddrawn off as gaseous pressurized nitrogen product (28, 31),characterized in that the evaporation space of the low-pressure-columntop condenser (7) is in the form of a forced-flow evaporator, thehigh-pressure column (4) has a barrier-plate section (8), which isarranged immediately above the point at which the feed air (3) isintroduced, and has one to five theoretical or practical plates, theoxygen-enriched liquid stream (11) which is introduced into thelow-pressure column (6) is withdrawn from the high-pressure column (4)above the barrier-plate section (8), a purge stream (9A) is withdrawnbelow the barrier-plate section (8) and removed (9B) from thedistillation column system, and the gaseous nitrogen stream (26A, 26B),before being warmed in the main heat exchanger (2), is warmed in acounter-current subcooler (12) in indirect heat exchange with theoxygen-enriched liquid stream (11) from the high-pressure column (4),and thus the fraction of air which is passed into the high-pressurecolumn in liquid form is reduced.
 2. Method according to claim 1,characterized in that the compressed, cleaned and cooled feed air (1) isintroduced (3) into the high-pressure column (4) in entirely gaseousform and is superheated in particular by at least 0.1 K or at least 0.2K.
 3. Method according to claim 1, characterized in that an oxygen-richliquid (15, 16) is withdrawn from the low-pressure column (6) and fed tothe evaporation space of the low-pressure-column top condenser (7), thegas generated in the evaporation space of the low-pressure-column topcondenser (7) is warmed as residual gas (32, 33) to an intermediatetemperature in the main heat exchanger (2) and subsequently (34)expanded in a work-performing manner in a residual-gas turbine (35), andthe residual gas (38, 40) expanded in a work-performing manner isreintroduced into the main heat exchanger (2) and warmed to aroundambient temperature.
 4. Method according to claim 3, characterized inthat the residual-gas turbine (35) is decelerated by a generator (36).5. Method according to claim 3, characterized in that the residual-gasturbine (35) is decelerated by a compressor (236) which compressesexpanded residual gas (39, 339) warmed to around ambient temperature,wherein the compressor is operated in particular in the warm state. 6.Method according to claim 1, characterized in that the evaporation spaceof the main condenser (5) is also in the form of a forced-flowevaporator.
 7. Method according to claim 1, characterized in that aliquid-nitrogen stream (22) is drawn off from the low-pressure column(6) or from the liquefaction space of the low-pressure-column topcondenser (7) and introduced into the high-pressure column (4) by meansof a pump (23).
 8. Method according to claim 1, characterized in that agaseous nitrogen stream (41) is drawn off from the low-pressure column(6) and obtained as a gaseous pressurized nitrogen product (PGAN, Seal).9. Method according to claim 1, characterized in that a liquid-nitrogenstream (22) is drawn off from the low-pressure column (6), warmed in thecounter-current subcooler (12) and drawn off as a liquid nitrogenproduct (125C, LIN).
 10. Apparatus for obtaining pressurized nitrogen bycryogenic separation of air with a distillation column system which hasa high-pressure column (4), a low-pressure column (6), and also a maincondenser (5) and a low-pressure-column top condenser (7), which areboth in the form of condenser-evaporators, having a main heat exchanger(2) for cooling compressed and cleaned feed air (1) and having means (3)for introducing feed air in gas form cooled in the main heat exchanger(2) into the high-pressure column (4), having means for withdrawing anoxygen-enriched liquid stream (11, 13) from the high-pressure column (4)and for introducing the oxygen-enriched liquid stream (11, 13) into thelow-pressure column, and having a product line for withdrawing a gaseousnitrogen stream (17, 26A, 26B, 27) from the high-pressure column (4) forwarming the gaseous nitrogen stream (17, 26A, 26B, 27) in the main heatexchanger (2) and for drawing off the warmed gaseous nitrogen stream(17, 26A, 26B, 27) as a gaseous pressurized nitrogen product (28, 31),characterized in that the evaporation space of the low-pressure-columntop condenser (7) is in the form of a forced-flow evaporator, thehigh-pressure column (4) has a barrier-plate section (8), which isarranged immediately above the point at which the feed air (3) isintroduced, and has one to five theoretical or practical plates, and themeans for withdrawing an oxygen-enriched liquid stream (11, 13) from thehigh-pressure column (4) are connected to the high-pressure column (4)above the barrier-plate section (8), wherein the apparatus also has apurge line for withdrawing a purge stream (9A) from the high-pressurecolumn (4) and for removing (9B) the purge stream from the distillationcolumn system, wherein the purge line is connected to the high-pressurecolumn (4) below the barrier-plate section (8), and a counter-currentsubcooler (12) for warming the gaseous nitrogen stream (26A, 26B) beforeit is warmed in the main heat exchanger (2) in indirect heat exchangewith the oxygen-enriched liquid stream (11) from the high-pressurecolumn (4).